$ F = \left[\begin{array}{rrr}1 & 1 & 1 \\ 1 & 1 & 0 \\ 1 & 2 & 0\end{array}\right]$ What is $ F^{-1}$ ?
$ = \left[\begin{array}{rrr}0 & 2 & -1 \\ 0 & -1 & 1 \\ 1 & -1 & 0\end{array}\right]$